Thermal insulation under vacuum



April 30, 1968 J. LECOMTE ET AL THERMAL INSULATION UNDER VACUUM FiledNov. 20. 1963 United States Patent 3,380,614 THERMAL INSULATION UNDERVACUUM Jean Lecomte, Paris, and Francois Gans, Gif-sur-Yvette, France,assignors to lAir Liquide, Societe Anonyrne Pour dEtude et lExploitationdes Procedes Georges Claude Filed Nov. 20, 1963, Ser. No. 325,021 Claimspriority, application France, Nov. 30, 1962, 917,160, Patent 1,348,589 4Claims. (Cl. 220--9) The present invention concerns heat insulationunder vacuum, including means of reflecting infra-red radiation, andintermediate means of a material of low heat conductibility, inparticular for containers of liquified gas at low temperature, such asliquid nitrogen, oxygen, hydrogen and helium.

It is known that for insulation maintained under vacuum to eliminate thetransmission of heat by gaseous convection an important fraction of theheat flux at low and very low temperature is provided by infra-redradiation from the warmer to the cooler wall of the insulation. Meanshave already been proposed for stopping this flux by arrangingreflecting means such as polished metal sheets or metal flakes, in itspath. These sheets or flakes must naturally be kept separated from eachother because of their high thermal conductibility. For this purposeeither thin sheets of fibres of low thermal conductibility, or a powderon a material possessing the same property are used. Insulation of thetype defined in US. patent application Ser. No. 196,986, filed May 23,1962, now Patent No. 3,218,816, and assigned to the assignee of thepresent application in which the sheets of insultaing fibres consist ofa fabric formed from these fibres, and in which there are empty spacesbetween the weft and war-p threads, has shown itself to be particularlyefficient.

Hitherto the material generally used for the thin sheets has been glass,by virtue of the ease of manufacture of glass fibres of very smalldiameter (a few microns or even less), For insulating powders, variousoxides such as silica gels, diatomaceous earth, or the natural silicateknown as perlite have been used. Although such thermal insulation allowsfor the storage of liquefied gases for several days with acceptablelosses by vaporization, the development of the use of extremely volatilegases such as hydrogen or helium in the liquid state, require thedevelopment of heat insulation still more eflicient than that alreadyknown, in order to keep the liquefied gases for long periods incontainers whose insulation is not to increase the volume excessively.

It has been discovered according to the invention that,

in contrast to what one would have expected, the residual thermalconduction in known insulation is largely due to the fact that thematerial of the intermediate means, such as glass, silica gel orperlite, has a high absorption coefficient for infra-red radiation.Although a consequence of this property is to reduce the transmission ofinfra-red radiation from a reflecting means situated in a relativelywarmer zone towards the adjacent reflecting means situated in arelatively cooler zone, it has been observed that this favourable effectwas more than compensated for by the undesirable effect due to thediminution in the reflection coefiicient of the reflecting means becauseof its contact with particles of a partially absorbing material 56ningeffect of the reflecting means).

According to the invention the thermal insulation is characterised inthat the material of the given intermediate means possesses a highinfra-red transmission coefficient, at least in the range of wavelengths corresponding to the heat emission in the temperature zone inwhich the intermediate means is situated.

FIGURE 1 is a graph showing transmission coefficient 'ice T in percentas a function of the infra-red wavelength, and

FIGURE 2 shows a structure of heat insulation material in adiagrammatic, schematic, cross-sectional view of a preferred embodimentof the invention.

Few materials exist which possess a high infra-red radiationtransmission coefficient through the full spectrum of wave-lengths. Itis however the case for polytetrafluoroethylene, generally sold underthe trademark Teflon.

This material is particularly suitable in the temperature zone between150 and K. In the temperature zone between ambient temperature and 250K., arsenic pentaselenide, As Se can also be used, and in the zonebetween 250 K. and 150 K., polyethylene or polystyrene may be used.

By way of illustration, FIG. 2 of the drawing shows a thermal insulationstructure made up by the stacking of thin sheets 4 of polishedaluminium, separated by sheets 5 each comprising apolytetrafluoroethylene fabric of thickness 0.16 mm. The average meshWidth of the fabric is 2 mm. and each thread of the fabric comprises 15elementary fibres of polytetrafluoroethylene of 18 microns diameter, thediameter of the thread obtained being 160 microns. The area density ofthe fabric obtained is 23.6 gm./m. The polytetrafluoroethylene used hadan infrared transmission coefficient, in the wavelength range between 25and 45 microns inclusive, of between 75 and (see graph in FIG. 1 showingthe transmission coefficient T (in percent) as a function of theinfra-red wavelength). Such insulation gives a heat transmissioncoeflicient appreciably less than an equivalent insulation in which theintermediate sheets are of a fabric of glass fibres of similar diameter,because of the reduction in the refleeting power of the screens due totheir contact with the glass fibres, which have a large absorptioncoeflicient for the radiation.

The thermal insulation structure thus far described is disposed in avacuum space 3 afforded by the inner wall 1 and the outer wall 2 of acontainer for liquefied gases.

What we claim is:

1. A heat insulating structure for containers for liquefied gases at lowtemperatures, comprising in an evacuated space between an inner and anouter wall, a plurality of reflective polished aluminum sheets spacedapart within said evacuated space and being substantially parallel tosaid container walls for reflecting infrared radiation, and intermediatespacing means comprising sheets of a woven fabric ofpolytetrafluoroethylene, in which there are empty spaces between theweft and the warp threads, said spacing means being disposed betweensaid aluminum sheets and between said aluminum sheets and at least oneof said container walls and in contiguous relation therewith, andpossessing a high infrared transmission coeflicient in the wavelengthrange for heat emission in the temperature zone in which saidintermediate means is situated.

2. The invention according to claim 1 in which said intermediate meansis effective from 75 K. to K.

3. A heat insulating structure for containers for liquefied gases at lowtemperatures, comprising in an evacuated space between an inner and anouter wall, a plurality of reflective polished aluminum sheets spacedapart within said evacuated space and being substantially parallel tosaid container walls for reflecting infrared radiation, and intermediatespacing means comprising sheets of a woven fabric of a polymer from thegroup consisting essentially of polyethylene and polystyrene, beingeffective from 150 K. to 250 K., in which there are empty spaces betweenthe weft and the war-p threads, said spacing means being disposedbetween said aluminum sheets and between said aluminum sheets and atleast one of said container walls and in contiguous relation therewith,and possessing a high infrared transmission coefiicient in thewavelength range for heat emission in the temperature Zone in which saidintermediate means is situated.

4. A heat insulating structure for containers for liquefied gases at alow temperatures, comprising in an evacuated space between an inner andan outer wall, a plurality of reflective polished aluminum sheets spacedapart within said evacuated space and being substantially parallel tosaid container walls for reflecting infrared radiation, and intermediatespacing means comprising sheets of a woven fabric of arsenicpentaselenide, being effective from 250 K. to ambient room temperature,in which there are empty spaces between the weft and the warp threads,said spacing means being disposed between said aluminum sheets andbetween said aluminum sheets and at least one of said container wallsand in contiguous relation therewith, and possessing a high infraredtransmission coefficient in the wavelength range for heat emission inthe temperature zone in which said intermediate means is situated.

References Cited UNITED STATES PATENTS 1,151,321 8/1915 Woodward.2,910,763 11/ 1959 Lauterbach. 3,007,596 11/1961 Matsch 220--9 3,018,0161/ 1962 Hnilieka. 3,199,715 8/1965 Paivanas 2209 2,804,886 9/1957 White.2,930,714 3/ 1960 Netherwood. 3,136,680 6/1964 Hachberg 161-189 FOREIGNPATENTS 488,767 12/ 1952 Canada. 1,264,507 5/ 1961 France.

THERON E. CONDON, Primary Examiner.

JAMES R. GARRETT, LOUIS G. MANCENE,

Examiners.

1. A HEAT INSULATING STRUCTURE FOR CONTAINERS FOR LIQUEFIED GASES AT LOWTEMPERATURES, COMPRISING IN AN EVACUATED SPACE BETWEEN AN INNER AND ANOUTER WALL, A PLURALITY OF REFLECTIVE POLISHED ALUMINUM SHEETS SPACEDAPART WITHIN SAID EVACUATED SPACE AND BEING SUBSTANTIALLY PARALLEL TOSAID CONTAINER WALLS FOR REFLECTING INFRARED RADIATION, AND INTERMEDIATESPACING MEANS COMPRISING SHEETS OF A WOVEN FABRIC OFPOLYTETRAFLUOROETHYLENE, IN WHICH THERE ARE EMPTY SPACES BETWEEN THEWEFT AND THE WARP THREADS, SAID SPACING MEANS BEING DISPOSED BETWEENSAID ALUMINUM SHEETS AND BETWEEN SAID ALUMINUM SHEETS AND AT LEAST ONEOF SAID CONTAINER WALLS AND IN CONTIGUOUS RELATION THEREWITH, ANDPOSSESSING A HIGH INFRARED TRANSMISSION COEFFICIENT IN THE WAVELENGTHRANGE FOR HEAT EMISSION IN THE TEMPERATURE ZONE IN WHICH SAIDINTERMEDIATE MEANS IS SITUATED.